These Agilent Varian HS-20 diffusion pump cold cap and nut follower (2 each), is part of the jet assembly and is a OEM Agilent Varian replacement part for the HS-20 diffusion vacuum pump. These are original OEM Agilent Varian replacement parts for the HS-20 diffusion pump having part number L8839301, see item #2 in the diagram exploded view & parts illustration below.